Biodegradable Alkali-Swellable Emulsion Polymers: Industrial and Commercial Thickeners.

Sustainability and circularity are key issues facing the global polymer industry. The search for biodegradable and environmentally-friendly polymers that can replace conventional materials is a difficult challenge that has been met with limited success. Alternatives must be cost-effective, scalable, and provide equivalent performance. We report that latexes made by the conventional emulsion polymerization of vinyl acetate and functional vinyl ester monomers are efficient thickeners for consumer products and biodegrade in wastewater. This approach uses readily-available starting materials and polymerization is carried out in water at room temperature, in one pot, and generates negligible waste. Moreover, the knowledge that poly(vinyl ester)s are biodegradable will lead to the design of new green polymer materials.

Synthesis and Biodegradation Studies of Low-Dispersity Poly(acrylic acid).

Poly(acrylic acid) (PAA) is produced on an industrial scale and widely-used in applications such as personal care products and cleaning formulations that end up "down-the-drain." Relatively high molecular weight PAA is considered poorly biodegradable, but little is known about the biodegradability of low molecular weight PAA at the wastewater treatment plant according to current regulatory and industrial Organization for Economic Co-operation and Development (OECD) standards. The synthesis, separation, and characterization of a series of ultralow dispersity PAA oligomers (i.e., D < 1.10) in the molecular weight range Mn ≈ 350-1200 Da and the results of biodegradability testing are reported. Miniaturized, high-throughput screening studies in a parallel respirometer reveals a strong trend toward lower biodegradation at higher molecular weight; these results are confirmed and expanded using standardized method OECD 301F. Biodegradability reaches ≈40% at Mn = 380 Da, ≈26% at Mn = 770 Da, and ≈17% at Mn = 1190 Da for discrete polyacid oligomers. These data not only shed light on potential biodegradation mechanisms for linear PAA, but also may inspire the future design of biodegradable PAA-containing macromolecules.

Emergence of Hexagonally Close-Packed Spheres in Linear Block Copolymer Melts.

The hexagonally close-packed (HCP) sphere phase is predicted to be stable across a narrow region of linear block copolymer phase space, but the small free energy difference separating it from face-centered cubic spheres usually results in phase coexistence. Here, we report the discovery of pure HCP spheres in linear block copolymer melts with A = poly(2,2,2-trifluoroethyl acrylate) ("F") and B = poly(2-dodecyl acrylate) ("2D") or poly(4-dodecyl acrylate) ("4D"). In 4DF diblocks and F4DF triblocks, the HCP phase emerges across a substantial range of A-block volume fractions (circa fA = 0.25-0.30), and in F4DF, it forms reversibly when subjected to various processing conditions which suggests an equilibrium state. The time scale associated with forming pure HCP upon quenching from a disordered liquid is intermediate to the ordering kinetics of the Frank-Kasper σ and A15 phases. However, unlike σ and A15, HCP nucleates directly from a supercooled liquid or soft solid without proceeding through an intermediate quasicrystal. Self-consistent field theory calculations indicate the stability of HCP is intimately tied to small amounts of molar mass dispersity (D); for example, an HCP-forming F4DF sample with fA = 0.27 has an experimentally measured D = 1.04. These insights challenge the conventional wisdom that pure HCP is difficult to access in linear block copolymer melts without the use of blending or other complex processing techniques.

Norbornadiene Chain-End Functional Polymers as Stable, Readily Available Precursors to Cyclopentadiene Derivatives.

A novel method for facile postpolymerization functionalization of synthetic polymers using terminal norbornadiene (NBD) building blocks is presented. Incorporation of the NBD functionality streamlines the synthesis of a wide array of block polymers utilizing multistep click chemistry strategies. Previously, the use of NBD-functionalized initiators produced polymers that underwent a cascade of Diels-Alder (DA) reactions to unveil a reactive cyclopentadiene (Cp) chain end. When coupled with a maleimide-bearing counterpart, a highly efficient DA cycloaddition with the terminal Cp can occur. To extend this concept to a range of polyacrylates and commercially available poly(ethylene glycol) systems, we developed a novel NBD acid building block for postpolymerization functionalization. Employing this process, we have demonstrated straightforward access to a library of block polymers that leverage this NBD click platform.

Rapid Generation of Block Copolymer Libraries Using Automated Chromatographic Separation.

A versatile and scalable strategy is reported for the rapid generation of block copolymer libraries spanning a wide range of compositions starting from a single parent copolymer. This strategy employs automated and operationally simple chromatographic separation that is demonstrated to be applicable to a variety of block copolymer chemistries on multigram scales with excellent mass recovery. The corresponding phase diagrams exhibit increased compositional resolution compared to those traditionally constructed via multiple, individual block copolymer syntheses. Increased uniformity and lower dispersity of the chromatographic libraries lead to differences in the location of order-order transitions and observable morphologies, highlighting the influence of dispersity on the self-assembly of block copolymers. Significantly, this separation technique greatly simplifies the exploration of block copolymer phase space across a range of compositions, monomer pairs, and molecular weights (up to 50000 amu), producing materials with increased control and homogeneity when compared to conventional strategies.

The development of peptide-boron difluoride formazanate conjugates as fluorescence imaging agents.

Two new fluorescence imaging probes have been synthesized by incorporating a versatile alkyne-substituted boron difluoride formazanate precursor with peptides through copper-catalyzed alkyne-azide cycloaddition. The formazanate dye was appended to a C-terminal amino acid of ghrelin for imaging the growth hormone secretagogue receptor (GHSR-1a). To demonstrate versatile bioconjugation chemistry, the formazanate dye was added to the N-terminus of bombesin for targeting the gastrin releasing peptide receptor (GRPR). These are the first examples of using this emerging class of dyes, boron difluoride formazanates, for the labelling of biomolecules.

Synthesis and Self-Assembly of ABn Miktoarm Star Polymers.

The stability of tetrahedrally close-packed (TCP) phases in block copolymer melts is predicted by theory to depend on molecular architecture, yet no experimental studies to date have probed its effect. Motivated by this open question, here we report an efficient synthesis of asymmetric ABn miktoarm star polymers using functionalized sugars as cores for orthogonal grafting-from block copolymerizations. A combination of ring-opening and atom transfer radical polymerization produced model low dispersity materials comprising a single A = poly(lactide) (L) and multiple B = poly(dodecyl acrylate) (D) arms that amplify "conformational asymmetry" through two concerted effects: the mikto architecture and disparate block statistical segment lengths. Analyzing the self-assembly of LD2 and LD3 samples resulted in the discovery of two TCP phases, σ and A15, that remained stable to significantly higher A-block volume fractions as the number of B arms increased. These results experimentally establish the importance of conformational asymmetry and molecular architecture as powerful design tools for the self-assembly of block copolymers into nonclassical phases.

Architecture Effects in Complex Spherical Assemblies of (AB)n-Type Block Copolymers.

Molecular architecture plays a key role in the self-assembly of block copolymers, but few studies have systematically examined the influence of chain connectivity on tetrahedrally close-packed (TCP) sphere phases. Here, we report a versatile material platform comprising two blocks with substantial conformational asymmetry, A = poly(trifluoroethyl acrylate) and B = poly(dodecyl acrylate), and use it to compare the phase behavior of AB diblocks, ABA triblocks, and (AB)n radial star copolymers with n = 3 or 4. Each architecture forms TCP sphere phases at minority A block compositions (fA < 0.5), namely, σ and A15, but with differences in the location of order-order phase boundaries that are not anticipated by mean-field self-consistent field theory simulations. These results expand the palette of polymer architectures that readily self-assemble into complex TCP structures and suggest important design considerations when targeting specific phases of interest.

Altering the optoelectronic properties of boron difluoride formazanate dyes via conjugation with platinum(II)-acetylides.

The combination of π-conjugated organic compounds and Pt(ii)-acetylides is a powerful strategy for the production of functional optoelectronic materials. The presence of the heavy element, Pt, in these compounds enhances electronic delocalization generally resulting in low-energy absorption and emission maxima and often leads to intersystem crossing, resulting in phosphorescence. When boron complexes of N-donor ligands, such as boron dipyrromethenes (BODIPYs), are involved the molecular and polymeric materials produced have properties that are advantageous for their use as oxygen-sensors, in triplet-triplet annihilation, and as the functional components of photovoltaics. Based on these exciting results, we endeavored to thoroughly examine the effect of Pt(ii)-acetylide conjugation on the properties of BF2 formazanate dyes, which offer improved redox properties and red-shifted absorption and emission bands compared to many structurally related BODIPYs. The results showed that phosphine-supported Pt(ii)-acetylide incorporation enhanced electronic conjugation, rendering the electrochemical reduction of the BF2 formazanate dyes more difficult, while also red-shifting their absorption and emission maxima. Unlike similar BODIPYs, the presence of Pt(ii) did not facilitate phosphorescence, but rather quenched fluorescence. This study provides significant insights into structure-property relationships and guiding principles for the design of BF2 formazanate dyes, a rapidly emerging family of readily accessible optoelectronic materials.

Stability of the A15 phase in diblock copolymer melts.

The self-assembly of block polymers into well-ordered nanostructures underpins their utility across fundamental and applied polymer science, yet only a handful of equilibrium morphologies are known with the simplest AB-type materials. Here, we report the discovery of the A15 sphere phase in single-component diblock copolymer melts comprising poly(dodecyl acrylate)-block-poly(lactide). A systematic exploration of phase space revealed that A15 forms across a substantial range of minority lactide block volume fractions (fL = 0.25 - 0.33) situated between the σ-sphere phase and hexagonally close-packed cylinders. Self-consistent field theory rationalizes the thermodynamic stability of A15 as a consequence of extreme conformational asymmetry. The experimentally observed A15-disorder phase transition is not captured using mean-field approximations but instead arises due to composition fluctuations as evidenced by fully fluctuating field-theoretic simulations. This combination of experiments and field-theoretic simulations provides rational design rules that can be used to generate unique, polymer-based mesophases through self-assembly.

A π-conjugated inorganic polymer constructed from boron difluoride formazanates and platinum(ii) diynes.

The first example of a π-conjugated polymer incorporating boron difluoride (BF2) formazanates is introduced. The film-forming properties, controllable reduction chemistry, and low optical band gap (ca. 1.4 eV) of the polymer make it an excellent candidate for use as a light-harvesting n-type semiconductor in organic electronics. Comparison of the polymer to model compounds confirmed that its unique optoelectronic properties can be directly attributed to the presence of the BF2 formazanate repeat unit and that the [Pt(PBu3)2]2+ unit must also be present to achieve the narrow band gaps observed.

Boron Difluoride Adducts of a Flexidentate Pyridine-Substituted Formazanate Ligand: Property Modulation via Protonation and Coordination Chemistry.

The synthesis and characterization of a flexidentate pyridine-substituted formazanate ligand and its boron difluoride adducts, formed via two different coordination modes of the title ligand, are described. The first adduct adopted a structure that was typical of other boron difluoride adducts of triarylformazanate ligands and contained a free pyridine subsituent, while the second was formed via the chelation of nitrogen atoms from the formazanate backbone and the pyridine substituent. Stepwise protonation of the pydridine-functionalized adduct, which is essentially nonemissive, resulted in a significant increase in the fluorescence quantum yield up to a maximum of 18%, prompting the study of this adduct as a pH sensor. The coordination chemistry of each adduct was explored through reactions with nickel(II) bromide [NiBr2(CH3CN)2], triflate [Ni(OTf)2], and 1,1,1,4,4,4-hexafluoroacetylacetonate [Ni(hfac)2(H2O)2] salts. Coordination to nickel(II) ions altered the physical properties of the boron difluoride formazanate adducts, including red-shifted absorption maxima and less negative reduction potentials. Together, these studies have demonstrated that the physical and electronic properties of boron difluoride adducts of formazanate ligands can be readily modulated through protonation and coordination chemistry.

Structurally Diverse Boron-Nitrogen Heterocycles from an N2 O23- Formazanate Ligand.

Five new compounds comprised of unprecedented boron-nitrogen heterocycles have been isolated from a single reaction of a potentially tetradentate N2 O23- formazanate ligand with BF3 ⋅OEt2 and NEt3 . Optimized yields for each product were obtained through variation of experimental conditions and rationalized in terms of relative Gibbs free energies of the products as determined by electronic structure calculations. Chemical reduction of two of these compounds resulted in the formation of a stable anion, radical anion, and diradical dianion. Structural and electronic properties of this new family of redox-active heterocycles were characterized using UV/vis absorption spectroscopy, cyclic voltammetry and X-ray crystallography.

Evaluation of Anisole-Substituted Boron Difluoride Formazanate Complexes for Fluorescence Cell Imaging.

Evaluation of three subclasses of boron difluoride formazanate complexes bearing o-, m-, and p-anisole N-aryl substituents (Ar) as readily accessible alternatives to boron dipyrromethene (BODIPY) dyes for cell imaging applications is described. While the wavelengths of maximum absorption (λmax ) and emission (λem ) observed for each subclass of complexes, which differed by their carbon-bound substituents (R), were similar, the emission quantum yields for 7 a-c (R=cyano) were enhanced relative to 8 a-c (R=nitro) and 9 a-c (R=phenyl). Complexes 7 a-c and 8 a-c were also significantly easier to reduce electrochemically to their radical anion and dianion forms compared to 9 a-c. Within each subclass, the o-substituted derivatives were more difficult to reduce, had shorter λmax and λem , and lower emission quantum yields than the p-substituted analogues as a result of sterically driven twisting of the N-aryl substituents and a decrease in the degree of π-conjugation. The m-substituted complexes were the least difficult to reduce and possessed intermediate λmax , λem , and quantum yields. The complexes studied also exhibited large Stokes shifts (82-152 nm, 2143-5483 cm(-1) ). Finally, the utility of complex 7 c (Ar=p-anisole, R=cyano), which can be prepared for just a few dollars per gram, for fluorescence cell imaging was demonstrated. The use of 7 c and 4',6-diamino-2-phenylindole (DAPI) allowed for simultaneous imaging of the cytoplasm and nucleus of mouse fibroblast cells.

Effect of Extended π Conjugation on the Spectroscopic and Electrochemical Properties of Boron Difluoride Formazanate Complexes.

The effect of extended π conjugation on the spectroscopic and electrochemical properties of boron difluoride (BF2) formazanate complexes was studied by the systematic comparison of phenyl- and naphthyl-substituted derivatives. Each of the BF2 complexes described was characterized by (1)H, (13)C, (11)B, and (19)F NMR spectroscopy, cyclic voltammetry, infrared spectroscopy, UV-vis absorption and emission spectroscopy, and mass spectrometry. X-ray crystallography and electronic structure calculations were used to rationalize the trends observed, including direct comparison of 3-cyano-, 3-nitro-, and 3-phenyl-substituted BF2 formazanate complexes. In all cases, the wavelengths of maximum absorption and emission were red-shifted as π conjugation was systematically extended (by replacing phenyl with naphthyl), fluorescence quantum yields increased (up to 10-fold), and electrochemical conversion of the formazanate complexes to their radical anion and dianion forms occurred at less negative potentials (easier to reduce).

Expanding the scope of strained-alkyne chemistry: a protection-deprotection strategy via the formation of a dicobalt-hexacarbonyl complex.

A protection-deprotection strategy for strained alkynes used for bioorthogonal chemistry is reported. A strained alkyne can be protected with dicobalt-octacarbonyl and we demonstrate for the first time that a strained alkyne can be re-formed and isolated under mild reaction conditions for further bioorthogonal reactivity. The protection-deprotection strategy herein reported will expand the versatility of strained alkynes for the preparation of substrates in chemical biology and materials applications.

Efficient electrochemiluminescence of a readily accessible boron difluoride formazanate dye.

Electrochemiluminescence (ECL) of a boron difluoride formazanate dye was investigated in the presence of tri-n-propylamine as a reductive co-reactant. The ECL mechanism was studied using ECL-voltage curves, spooling ECL, and accumulative ECL spectroscopy. The ECL occurs at 724 nm by three distinct, voltage-dependent mechanisms of light emission.

Substituent-dependent optical and electrochemical properties of triarylformazanate boron difluoride complexes.

The straightforward synthesis and detailed characterization of nine substituted triarylformazanate boron difluoride complexes is reported. The effect of electron-donating (p-anisole) and electron-withdrawing (p-benzonitrile) substituents on optical and electrochemical properties, relative to phenyl substituents, was studied at two different positions within the formazanate ligand framework. Each of the BF2 complexes was characterized by (1)H, (13)C, (11)B, and (19)F NMR spectroscopy, cyclic voltammetry, infrared spectroscopy, UV-vis absorption and emission spectroscopy, mass spectrometry, and elemental analysis. Select examples were studied by X-ray crystallography, revealing highly delocalized structures in the solid state. The complexes were reversibly reduced in two steps electrochemically to their radical anion and dianion forms. The complexes also exhibited substituent-dependent absorption and emission properties, accompanied by significant Stokes shifts, with the aryl substituents at the 1,5-positions of the formazanate backbone having a greater influence on these properties than aryl substituents at the 3-position. Breaking the symmetry in three different complexes resulted in a modest increase in emission intensity relative to that of symmetrically substituted derivatives.

Structurally tunable 3-cyanoformazanate boron difluoride dyes.

The straightforward synthesis of a series of 3-cyanoformazanate boron difluoride dyes is reported. Phenyl, 4-methoxyphenyl and 4-cyanophenyl N-substituted derivatives were isolated and characterized by single-crystal X-ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. The compounds were demonstrated to possess tunable, substituent-dependent absorption, emission, and electrochemical properties, which were rationalized through electronic structure calculations.

Hydrogen-bond-supported dimeric boron complexes of potentially tetradentate ß-diketiminate ligands.

Two dimeric boron complexes of potentially tetradentate and trianionic ß-diketiminate ligands bearing phenol substituents were prepared and characterized. The synthetic routes employed were designed to circumvent the undesirable formation of ß-ketimines and 2-methylbenzoxazoles observed when traditional synthetic routes toward the target ß-diketiminate ligands were attempted. The title complexes were isolated via demethylation of ß-diketimine ligands and boron difluoride complexes bearing 2-anisole N-aryl substituents using boron tribromide. The resulting complexes were found to contain a unique hydrogen-bond-supported boron-oxygen-boron bridge, as confirmed by X-ray crystallography. The stability of the resulting dimeric structures relative to the corresponding monomeric, tetradentate boron complexes was studied computationally, and theory confirmed that the dimeric structures were strongly favored. The absorption spectra of the dimers were red-shifted relative to the parent ß-diketimine ligands. The complexes were irreversibly oxidized and reduced electrochemically and were weakly emissive at low concentrations (Stokes shifts between 23 and 31 nm), showing little solvent dependence.